In recent years, a fuel cell has been noticed as a vehicle drive source and a stationary power source in response to social need and trend against the background of energy and environmental problems. The fuel cell is classified into various types in accordance with kinds of electrolytes and kinds of electrodes, and typical examples thereof include an alkali type, a phosphoric acid type, a molten carbonate type, a solid electrolyte type and a polymer electrolyte type. Among them, a polymer electrolyte fuel cell (PEFC) operatable at a low temperature (ordinarily 100° C. or less) has been noticed; in recent years, the development and practical use thereof have been advanced as a low-pollution power source for automobiles.
Generally, the configuration of PEFC is a structure such that a membrane electrode assembly (MEA) is sandwiched between separators. Generally, MEA has a structure such that a gas diffusion layer, a cathode catalyst layer, a solid polymer electrolyte membrane, an anode catalyst layer and a gas diffusion layer are laminated.
The following electrochemical reaction proceeds in MEA. First, hydrogen contained in fuel gas supplied to the anode (fuel electrode) side is oxidized by the catalyst to become a proton and an electron. Next, the produced proton reaches the cathode (air electrode) catalyst layer through a polymer electrolyte contained in the anode catalyst layer and the solid polymer electrolyte membrane contacting with the anode catalyst layer. Also, the electron produced in the anode catalyst layer reaches the cathode catalyst layer through a conductive carrier composing the anode catalyst layer, the gas diffusion layer contacting with the opposite side of the anode catalyst layer to the solid polymer electrolyte membrane, a gas separator, and an external circuit. Then, the proton and the electron, which reached the cathode catalyst layer, react with oxygen contained in oxidizing gas supplied to the cathode catalyst layer to produce water. In the fuel cell, electricity may be taken out to the exterior through the above-mentioned electrochemical reaction.
A vehicle drive source and a stationary power source have been studied as the uses of PEFC, and durability over a long term is demanded for the application to these uses. Above all, in the case of being used as the vehicle drive source, it is demanded that cell characteristics be not deteriorated due to frequent operation stops.
In particular, in a catalyst comprising platinum (Pt) or platinum alloy, a carbon material such as carbon black for supporting the catalyst, and an electrode catalyst layer containing a proton conductive polymer electrolyte, corrosion of the carbon material and decomposition degradation of the polymer electrolyte are easily caused by the repetition of operation stops. Thus, gas diffusivity and drainage of the electrode are deteriorated, concentration overvoltage is increased, and a tendency to deteriorate cell characteristics is brought.
On the contrary, a method for improving corrosion resistance of the catalyst layer by using the carbon material as a carrier, such that heat treatment controls crystallinity of the carbon material and improves corrosion resistance thereof, is known. However, such heat treatment brings an improvement in durability, whereas a decrease in specific surface area of the carbon material and pore (primary pore) capacity is brought. Thus, in the catalyst layer using the heat-treated carbon carrier, the problem is that the surface area effective for a reaction is decreased and power generation efficiency is deteriorated.
Then, many attempts to intend compatibility between durability of the catalyst layer and power generation performance have been made. For example, in Patent Document 1, a method for controlling a pore in the electrode catalyst layer is disclosed. In this method, a pore capacity of 0.01 to 2.0 μm in the electrode catalyst layer is controlled to 3.8 μl/cm2/mg-Pt or more, and a pore capacity of 0.01 to 0.15 μm is controlled to 2.0 μl/cm2/mg-Pt or more. The pore of 0.01 to 0.15 μm is intended for mainly supplying fuel gas and oxidative gas, and the pore of 0.15 to 2.0 μm is intended for discharging water produced by power generation. It is conceived that the control of these pores allows both excellent durability and excellent power generation performance to be offered.